Though related, this is no longer true. You can now purchase a lvl 90 character boost. Some power levelers have hoped for a day when you could use real money to buy individual items in-game, some times called "micro-transactions." At a Game Developers Conference 2008, Rob Pardo gave a strong indication, Blizzard would not ever go this route with World of Warcraft.

Try to gather up all quests for a dungeon before trying to queue up for it. Preferably pick up dungeon quests as soon as they are available to avoid getting them when you are over leveled. Dungeons themselves reward a good amount of experience due to the grouping of mobs, but the quests on top of that reward great experience and decent questing gear.
It still will be but it takes up an Azerite armor slot. ST is not as important as AoE when leveling because you want to grab a pack of mobs, burst them down, loot and repeat. For DK specifically, Unholy is better in my opinion for leveling because it has a lot more aoe than Frost with epidemic. Unholy was my second fastest leveling time and I did it on a pre-made character with no leggos or consumables.

Full disclosure on my time with Battle for Azeroth thus far: I leveled a Horde character to 120 and completed all three Zandalar storylines plus put about 20 hours into end-game content. I have also leveled an Alliance character partway through Kul Tiras and completed portions of the Stormsong Valley and Tiragarde Sound storylines. My only experience in Drustavar has been through the eyes of the Horde thus far.

World of Warcraft’s Legion expansion has been unleashed on the live servers and players are starting to arrive at The Broken Isles to plunder its untouched reserves of gold. Some are diving into the new dungeons and raids while others are undertaking the array of quests in the vast region. See how you, too, can gain immense amounts of gold in this exciting expansion.

Just before Battle for Azeroth goes live, you will want to log all of your characters out in the zone of Silithus so that you can acquire the quest to talk to Magni and obtain your Heart of Azeroth (refer to our Heart of Azeroth guide for more information about this item). This is the new artifact-type of piece of equipment that you will use for all of the Battle for Azeroth expansion. Once you acquire your permanent neck slot piece of equipment, get back to Stormwind or Orgrimmar to begin your scenario that leads into the introductory areas of Kul Tiras and Zandalar.
World of Warcraft’s Legion expansion has been unleashed on the live servers and players are starting to arrive at The Broken Isles to plunder its untouched reserves of gold. Some are diving into the new dungeons and raids while others are undertaking the array of quests in the vast region. See how you, too, can gain immense amounts of gold in this exciting expansion.
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.
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Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide